Jisho

I meant "sea" not "i"

P.s. : @kimtaro please implement a edit/delete feature for forums :)

Yes, the origin of the word is みず and うみ.

At its origin, うみ meant literally any large accumulation of water, from ponds and swamps and beyond. Over time, more distinctions got made, we ended up with a situation where only large saltwater bodies of water are called うみ.

Before that time where was a point where they were calling lakes みずうみ, and seas or oceans しおうみ (or しほうみ if you consider classical spelling). しお is the word for both "salt" and "tide." The word しおうみ didn't carry on to modern Japanese as far as I can tell, and うみ simply became the word to express that idea.

Here's a resource on the origin in Japanese.
https://gogen-yurai.jp/mizuumi/